This invention relates to a photoreceptor, particularly to an electrophotographic photoreceptor.
In the prior art, as electrophotographic photoreceptors, inorganic photoreceptors having photosensitive layers composed mainly of inorganic photosensitive materials such as selenium, zinc oxide, cadmium sulfide, etc., have been widely used.
On the other hand, it has been actively developed and studied in recent years to utilize various organic photoconductive materials as the material for the photosensitive layers of electrophotographic photoreceptors.
For example, Japanese patent publication No. 10496/1975 discloses an organic photoreceptor having a photosensitive layer containing poly-N-vinyl carbazole and 2,4,7-trinitro-9-fluorenone. However, this photoreceptor is not necessarily satifactory in sensitivity and durability. For improvement of such drawbacks, it has been attempted to develop an organic photosensitve member having high sensitivity and great durability by permitting different substances to be individually responsible for the carrier generation function and carrier transport function in the photosensitive layer. In such function separation type electrophotographic photoreceptor so to speak, since substances exhibiting various functions can be selected from a wide scope of materials, it is possible to prepare an electrophotographic photoreceptor having any desired characteristic with relative ease.
As the carrier generation substance effective for such a function separation type electrophotographic photoreceptor, a large number of substances have been proposed in the prior art. A typical example of using an inorganic substance is amorphous selenium as disclosed in Japanese patent publication No. 16198/1968. This is combined with an organic carrier transport substance.
Also, a large number of electrophotographic photoreceptors using organic dyes or organic pigments as the carrier generation substance have been proposed. For example, those having photosensitive layers containing bisazo compounds have been already known from Japanese provisional patent publications No. 37543/1972, No. 22834/1980, No. 79632/1979 and No. 116040/1981.
Whereas, the known photoreceptors using organic photoconductive materials are generally used for negative charging. This is because use of negative charging is advantageous in aspect of photosensitivity, etc., since mobility of the holes of the carriers is great.
However, by use of such negative charging, the following problems have been clarified to be involved. That is, the first problem is that ozone will be readily generated in the atmosphere during negative charging by a charger to worsen the environmental condition. Another problem is the positively chargeable toner which is required for developing the photoreceptor for negative charging, and the positively chargeable toner can be prepared with difficulty in view of the triboelectric charging series for ferromagnetic carrier particles.
Accordingly, it has been proposed to use a photoreceptor by use of an organic photoconductive material for positive charging. For example, in the case of a photoreceptor for positive charging in which a carrier transporting layer is laminated on a carrier generating layer and the carrier transporting layer is formed of a substance with great electron transporting ability, trinitrofluorenone, etc., is contained in the carrier transporting layer, but this substance is not suitable because of having carcinogenicity. On the other hand, it may be conceivable to give a photoreceptor for positive charging having a carrier generating layer laminated on a carrier transporting layer having a great hole transporting ability, but this becomes worsened in printing resistance, etc., due to the presence of a very thin carrier generating layer on the surface side and therefore it is not a practical layer constitution.
Also, as a photoreceptor for positive charging, U.S. Pat. No. 3,615,414 disclosed one having a thiapyrylium salt (carrier generation substance) incorporated so as to form a eutectic complex with a polycarbonate (binder resin). However, such a known photoreceptor has the drawbacks that the memory phenomenon is great and also that the ghost is liable to be generated. Also, in U.S. Pat. No. 3,357,989, a photoreceptor in which phthalocyanine is to be contained is disclosed. However, phthalocyanine is changed in characteristics depending on the crystal form and the crystal form is required to be strictly controlled. Further, it is deficient in short wavelength sensitivity and also great in the memory phenomenon, and therefore not suitable for a copying machine using a light source in the visible light wavelength region.
Under the state of the art as described above, it has been hardly realizable to use a photoreceptor by use of an organic photoconductive material for positive charging, and therefore, it has been used exclusively for negative charging.
Generally speaking, substances capable of generating carriers by absorption of visible light can hardly themselves form films except for only a part of them such as amorphous selenium, etc., and yet have the drawback of having poor retentive force for the charges given to the surface. On the contrary, substances which are excellent in film forming ability and can retain charges of 500 V or higher for a long time with a thickness of about 10 .mu.m, have the drawback that they have generally no sufficient photoconductivity by absortion of visible light.
For such reasons, as shown in FIG. 1, it has been proposed to form a laminated product 4 by providing a carrier generating layer 2 containing a substance capable of generating charged carriers by absorption of visible light through a subbing layer 5 on an electroconductive substrate 1 such as of Aluminum, etc., and further providing a carrier transporting layer 3 for transporting either one or both of positive and negative charged carriers generated in the carrier generating layer and constitute a photosensitive layer with this laminated product. Thus, by permitting separate substances to be responsible for generation and transport of charged carriers, the selection scope of the materials can be broadened and the various characteristics demanded in electrophotographic process such as charge retentive force, surface strength, sensitivity to visible light and stability during repeated uses, etc., can be enhanced or improved.
In FIG. 1, a constitution having the carrier generating layer 2 provided on the carrier transporting layer 3 may be also alternatively used.
In such a photoreceptor, U.S. Pat. No. 4,431,722 shows to use a brominated anthanthrone pigment as a high grade organic pigment in the photosensitive layer, particularly the carrier generating layer. This pigment gives higher sensitivity as compared with the case of inorganic particles or a perylene type pigment of the prior art and can provide a photoreceptor which is uniform and has good scratch property.
However, in order to use this pigment for an electrophotographic photoreceptor, both of its crystallinity and purity are required to be enhanced and the sublimation purification method is effective for realizing this requirement. However, according to the investigations made by the present inventors, in the case of the prior art method, when the pigment obtained is pulverized and dispersed to form a coating material for photoreceptor, if the share stress applied on the pigment is great (when the substrate temperature is high, and the pigment particles are great and highly crystalline), crystal latices may be deformed or impurities are liable to be entrained from a dispersing vessel. For this reason, charging ability and sensitivity tend to be lowered, and the quantity of lowering in repeated potential to be increased. On the contrary, when the shear stress applied on the pigment is small (in this case, the substrate temperature is low, and the pigment particles are small and low crystalline), electrophotographic characteristics are unsuitably deteriorated.